[3.13] gh-145856: Fix plistlib.dump() with skipkeys=True and sort_keys=True (GH-150109) #157737

A dict with non-string keys raised TypeError, because the keys were sorted
before skipping non-string keys.
(cherry picked from commit 7f7ff4c99d1f53424be8c323c50f3623c3488ba1)

Co-authored-by: ankhikarmakar [email protected]
Co-authored-by: VanshAgarwal24036 [email protected]
Co-authored-by: Serhiy Storchaka [email protected]
Co-authored-by: Claude Opus 5 (1M context) [email protected]

• Issue: gh-145856
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